Many types of switches which are activated and deactivated with angular movement are well known to those skilled in the art. Perhaps the most common of these switches is the mercury tilt switch which uses a globule of mercury in a sealed vial to conductively bridge two contacts. As the vial is tilted, the mercury moves from one end of the vial to the other end thereby moving in and out of engagement with the contacts.
Mercury tilt switches have been effectively used for years in many applications. However, due to the toxicity of mercury these switches have been disfavored where severe applications involving vibration, thermal changes, and sever shock are expected. In fact, many automobile manufacturers now require that the lamp assemblies used in their automobiles not contain mercury in view of the environmental risks involved.
Accordingly, the art has turned to several alternatives. One solution is disclosed in U.S. Pat. No. 5,155,308 ("the '308 patent") to Blair which issued on Oct. 13, 1992. The device taught in the '308 patent uses a rolling sphere which is movable on a horizontal planar surface or lever. The lever is placed on a fulcrum point fixed to a support member. Movement of the sphere on the lever causes engagement and disengagement of contacts located on the lever and the support member to complete a circuit.
Several disadvantages associated with the design of the '308 patent have been recognized, particularly if the design were to be used in severe applications. First, a dual circuit point contact arrangement as used in the '308 patent is susceptible to contamination resulting from vibration or arcing which reduces the useful life of the switch. Vibration and contamination can also affect the flow of current through the fulcrum point on the support member, and cause contact wear or chatter between the contacts on the lever and support member.
In addition, the movement of the lever in relation to the support member causes the contact placed on the lever to be forced down on the support member contact in an angular relationship. This wiping down of one contact on the other results in friction between the contacts causing them to stick upon reverse movement of the sphere. As a result, the angular differential between engagement and disengagement of the switch is increased.
Finally, the device disclosed in the '308 patent must be integrated into the housing of a lamp assembly if it is to be used in a lighting device. This complicates the assembly and adjustment processes, and significantly adds to the expense of a lamp assembly incorporating the switch.